High-pressure sodium lamps are well-known in the lighting field, and are currently in wide use by many city utilities for street lighting purposes. As a person skilled in the art would know, although such lamps have a long lifespan, they eventually fail over time because their sodium becomes depleted to a point where lamp voltage can no longer maintain a continuous arc. The result is a cycling condition where a depleted lamp continually flashes or attempts to start. Not only is this a difficult condition to detect and correct both quickly and cost-effectively, but it is also annoying, especially in residential areas where it can be visually distracting and/or cause radio and television interference.
FIG. 6 herein, which is labeled "prior art", schematically illustrates the start-up operation of a typical high-pressure sodium lamp. Each lamp is normally powered by a line voltage of 120 volts AC, which is schematically indicated at 1, 3. A photocell sensor control 5, positioned in series between the power source and the lamp, is operative to supply power at dusk, and to cut off power at dawn.
In the evening, when the photosensor control 5 initially causes power to be supplied, the lamp is initially in an unlit condition. Such lamps have a ballast choke/transformer 7 with a secondary winding or coil 9 that is connected to a pulsing starter device 11. When power is initially supplied, the starter device 11 sends pulses to the secondary coil 9. This causes the ballast 7 to act as a step-up transformer that generates high voltage spikes of several thousand volts across the lamp's electrodes 13, 15, and consequently results in ignition of the lamp. Once ignition occurs, current flow through the ballast causes the lamp voltage to drop (typically from about 150 to 55 volts AC), and pulsing from the starter device 11 ends. If the lamp cannot hold ignition because of sodium depletion, it will subsequently and repetitively attempt to restart as soon as it cools sufficiently so that sodium ionization can once again take place.
Obviously, cycling is correctable by simply replacing a depleted lamp. However, if a cycling condition is allowed to continue over a period of time, it eventually damages the lamp's starter/ballast unit 7, 11, typically by burning out the ballast 11. When this happens, the lamp ceases to cycle, but the starter/ballast unit must then be replaced along with the depleted lamp, resulting in higher overall costs of repair. For such reason, it is important to detect a cycling condition as soon as possible.
From the standpoint of labor, many or most city utilities have no cost-effective means for quickly detecting when such lamps are cycling. The typical utility does not have service people checking street lamps at night, which is the only time cycling is apparent since such lamps normally do not operate during the day.
Furthermore, cycling is difficult to detect even in situations where service checks are made at night. Depending on the level of sodium depletion, a cycling lamp often remains lit several minutes or more before it loses its arc and attempts to relight. This requires a service person to visually monitor individual lamps for more than just a brief period of time in order to discover whether cycling is occurring.
Since high-pressure sodium lamps have a predicted service life, most city utilities have simply taken to automatically replacing groups of lamps at selected times after they have been placed in service, regardless of whether or not a significant number of such lamps have actually begun to cycle. This is inefficient because it too often results in an earlier than necessary lamp replacement, or replacement after many lamp ballasts have already burned out, and consequently, does not make optimum use of each lamp.
Historically, high-pressure sodium lamps went into large-scale result of the energy shortages created by an Arab oil embargo in or about that time. High-pressure sodium lamps have approximately twice the energy efficiency of their predecessors, mercury vapor lamps, which were the most common street lamps in use before that time. The sodium lamps put into service in the mid-70's are now reaching the end of their design life. This means that the above-described cycling problem is becoming pressing, and must be quickly solved in a way that will maximize the life of existing lamps in an easy-to-implement, cost-effective manner.
The patent literature discloses that few inventors or compares have yet had occasion to address the above problem. One notable exception involves the efforts of Area Lighting Research, a Hackettstown, N.J. company. Area Lighting is the assignee of two U.S. patents, one issued on June 10, 1980 to Duve et al. (U.S. Pat. No. 4,207,500), and the other issued on Sept. 25, 1984 to Lindner et al. (U.S. Pat. No. 4,473,779). Both patents specifically relate to the cycling malfunction of depleted high-pressure sodium lamps, and each offers a solution, albeit one that is different from the invention disclosed here. It should be mentioned in passing that both patents provide a much more detailed description of the cause of the cycling malfunction than the cursory explanation provided above. Accordingly, their disclosures are incorporated herein by reference.
Duve et al. discloses a cut-off device that activates a relay in response to a signal from a detector-signal generator that senses when the voltage increase across the lamp is greater in magnitude than the lamp's normal operating voltage. The increase in voltage corresponds to the lamp's attempt to relight itself. A timing circuit monitors the signal from the detector-signal generator, and determines whether the sensed increase in voltage constitutes undesirable cycling. If so, the timing circuit activates the relay, thus cutting off power to the lamp.
Lindner et al. claims to be an improvement over Duve, and determines cycling by sensing a change in lamp power factor. In doing so, Lindner uses the combination of both a voltage signal generator and a current signal generator which simultaneously transmit their signals to a comparator-processor, where the latter compares their phases. When their phases have a certain known relationship that corresponds to cycling, Lindner similarly activates a relay cutting off power to the lamp.
As will become apparent, the present invention provides an anti-cycling device that is simpler in both design and operation than either one of the two devices discussed above. Further, the device disclosed here is low in cost, extremely reliable, and is equally well-suited for either retrofitting to street lamps presently in use, or factory installation by the lamp manufacturer.